This invention relates generally to heaters and more specifically electric heaters employing heat transfer members used to heat fluids such as air. By way of example, compartment heaters for vehicles have used heat exchange fin members mounted in air passages with self-regulating electrical resistance heater elements of positive temperature coefficient of resistivity (PTC) mounted in heat transfer relation with the heat exchange fin members. Upon energization the self-regulating heater elements quickly increase in temperature to a so called anomaly temperature and they in turn heat up the fin members. Air directed through the passage is then heated by the heat exchange fin members. PTC heating is advantageous for several reasons. For example, such heaters are characterized by having fast warm up due to the heater material's low initial resistance yet the heaters do not overshoot, that is, the temperature of the heater is limited essentially at its anomaly temperature. PTC heating provides high output at low temperature and high airflow while maintaining a relatively constant operating temperature. Such heaters are of rugged construction compatible with high vibration and high temperature automotive environments, create no electrical or mechanical noise and have wide voltage range capability. An example of a heater of this type is shown and described in U.S. Pat. 5,256,857 issued Oct. 26, 1993, assigned to the assignee of the present invention.
Although there are no minimum flow requirements for PTC heaters as there would be for a conventional straight resistance heater which would overheat and burn-up under still air conditions, if the heater is proximate to materials which are sensitive to high temperatures, for example, low temperature plastic in the duct near the heater, then generally some type of control device is employed to de-energize the heater when air flow is below some minimum rate.
It is an object of the present invention to provide a novel and improved heater for heating a fluid stream, to provide a heater particularly adapted for use for heating an air stream for an automotive passenger compartment or the like, to provide such a heater which is inexpensive to manufacture and assemble, to provide such a heater which is reliable, safe and efficient to use and to provide a heater which is more adaptable for use with temperature sensitive materials located proximate to the heater than prior art PTC heaters. Yet another object is the provision of a fluid stream heater which is particularly useful with high voltage electrically powered vehicles.